Pneumatic rock drills generally include a housing, in which is formed a cylindrical chamber, and a reciprocating hammer is mounted in the chamber. An anvil or shank at one end of the chamber is positioned to be struck by the hammer. Pressurized air is supplied to the cylinder alternately on opposite sides of the hammer to cause the hammer to reciprocate in the cylinder and to repeatedly strike the shank.
In some applications, several drill rods may be connected in a string between the hammer and a drill bit to reach the bottom of a hole being drilled. The rods are joined axially and the impact from the hammer is transmitted along the string of rods to the drill bit. This arrangement is typically used in drilling blast holes in rock, which requires holes that are relatively long and narrow.
For reasons related to safety, the handling characteristics of the drill apparatus, and the cost of air compressors, pneumatic drills generally use relatively low pressure air, typically in the range of 60 to 100 p.s.i. In order to generate the forces necessary for rock drilling from relatively low pressure air, pneumatic drills of the prior art have typically been provided with large chamber bores and large diameter hammer piston heads. Typical of this type of drill are the Gardner-Denver PR1000, PR66, and PR80 Drills. In these drills, a drill hammer has a substantially larger cross-sectional diameter at the hammer piston head than does the remainder of the length of the hammer and the impact face of the hammer. The large cross-section piston head has a large surface area against which relatively low pressure air acts to develop forces to accelerate the hammer.
Kinetic energy from movement of the hammer is transformed into impact energy when the hammer strikes the shank and an incident wave form resulting from particle motion in the hammer is generated. At each interface between components of the drill string, such as at the interface between the hammer and the shank, the shank and any drill rods, the drill rods or the shank and a drill bit, and the drill bit and the rock being drilled, the incident wave form gives rise to transmitted and reflected wave form components, which propagate back and forth in the drill string. The drill bit is pushed into the rock when the leading compressive part of the original incident wave form reaches the drill bit.
The length and shape of the incident wave form is a function of drill string geometry; particularly, the length and diameter of a drill hammer, a shank, drill rods in a string, and a drill bit. The amplitude of the stress component of the incident wave form is largely a function of impact velocity.
Conventional pneumatic hammer configurations present problems in the efficient formation of kinetic energy of the moving hammer into impact energy in the stationary shank, and in the efficient transfer of impact energy along the drill string to the drill bit. The reflected wave form component generated at each interface is a function of the impedance or dynamic stiffness at the interface. A portion of the compressive tail of the reflected wave comprises rebound losses back into the preceding impacting member. The rebound portion of the reflected wave form is larger with a stiff response, but could be equal to zero for a free end reflection.
Although the reflected waves may themselves be reflected in the rods, and may eventually reach the drill bit, reflected energy generally performs little work on the rock and may be considered lost. Energy is lost, for example, to friction in the drill string couplings as a stress wave passes through them. The energy transfer from the first rod to the coupling, and from the coupling to the next rod in the string causes unbalanced tensile and compressive forces in the coupling. The unbalanced forces result in movement between the couplings and the rods which give rise to friction losses. A significant amount of these energy losses may be attributed to reflected energy. Thus, to achieve efficient transfer of the energy of impact to a drill bit, it is desirable to minimize reflected energy.
The reflected energy in hammer drills can be a significant portion of the total energy generated during the impact. As is explained by B. Lundberg, Some Basic Problems in Percussive Rock Destruction, 214-15 (1971), the reflected wave component is minimized in drilling apparatus in which the impedance or dynamic stiffness between the hammer, successive drilling apparatus components, and the rock being drilled are equal. In impact devices in which the hammer, the shanks, the drill rods, and the drill bit are formed of the same or similar materials (i.e., the material densities and wave velocity through the materials are substantially the same) the reflected wave component is minimized when the cross-sectional areas of the hammer and the shanks taken through planes at any point on a longitudinal axis through the hammer or rods are equal.
The theoretical stress wave of a conventional hammer configuration is illustrated in FIGS. 1A, 1B, and 1C by wave forms X', X" and X'". The stress amplitude a of the first stress component of the wave form is related to the hammer geometry, as shown by the equation: ##EQU1## where: V.sub.i is the velocity of the hammer at impact; A.sub.1 is the cross-sectional area of the hammer;
A.sub.2 is the cross-sectional area of the shank; PA1 E is Young's modulus; PA1 c is the wave velocity in the hammer material; and PA1 .sigma. is the stress amplitude. PA1 A equals the area of the drill string member at the point of measurement.
The stress amplitude-time curve illustrated by the wave forms X', X" and X'" in FIGS. 1A, 1B, and 1C is characteristic of a conventional hammer having a larger cross-sectional area (A.sub.1) at a piston head than over the rest of the hammer body. The resulting wave forms are comprised of numerous transmission and reflection components. The sharp stress amplitude peaks P', P" and P'" in the wave forms is the result of wave reflection at the large cross-section of the conventional hammer geometry. FIGS. 1A, 1B, and 1C further illustrate that, for a particular hammer velocity, the portions of the stress amplitude wave attributable to other cross-sectional areas of the hammer are at a minimum where the cross-sectional area of the hammer is at a minimum, i.e, over the length of an otherwise constant cross-sectional area hammer having a smaller cross-sectional area (A.sub.1) than the piston head.
The amplitude of the reflected wave and the various losses due to rebound and friction are functions of the amplitude of the incident stress wave generated during impact and the stiffness response characteristics at the various interfaces in the apparatus. Reducing the amplitude of the incident wave attributable to different cross-sectional areas over the length of a hammer or a shank or other component of the drill string and optimizing stiffness response characteristics reduces these energy losses.
The area under the curves in FIGS. 1A, 1B, and 1C represents the incident impulse, and may be expressed as: EQU I=Ax.intg..sigma.dt
The total energy content of the stress wave is expressed as: ##EQU2##
FIGS. 1A, 1B, and 1C also illustrate theoretical stress wave forms Y', Y" and Y'". Waves Y', Y", and Y'" are more rectangular in shape than waves X', X", and X'" and peaks in the stress amplitude have been minimized. The energy content of each wave Y may be the same as the energy content of each wave X. Conventional hammers typically produce most of the available energy in the first part of a wave form and produce a tail of relatively low stress amplitude over the remainder of the wave form. The relatively constant stress amplitude transfer of wave B is associated with hammer and drill components having constant cross-sectional areas over their length. It is possible to minimize reflected wave energy losses where it is possible to generate such a wave form by forming the hammer and drill components such that they are, individually, of constant cross-sectional area, and such that the cross-sectional areas of the hammer and the drill components are equal to one another.
Although wave forms Y produced by an apparatus including hammer and drill components of constant and equal cross-sectional area may have a lower peak stress amplitude than wave forms X produced with conventional pneumatic apparatus having sharp peaks P, the wave form Y may nevertheless contain the same amount of energy as, or more energy than the wave form X because energy transfer can occur over a longer period of time. Further, the energy content of each wave form, X and Y, is limited by the stress that the hammer or the drill string components can sustain. Therefore, an apparatus including hammer and drill components of constant and equal cross-sectional area in which the peak stress amplitude of the wave form Y is substantially equal to the maximum stress (plus a safety factor) that the hammer or drill string components can sustain, and for which energy transfer at the peak stress amplitude occurs over a somewhat extended period of time, can transfer more energy than a conventional pneumatic apparatus including hammer and drill string components for which the peak stress amplitude P of the wave form X is substantially equal to the maximum stress (plus a safety factor) that the conventional hammer and drill string components can sustain, and for which energy transfer at the peak stress amplitude is comparatively brief.
Hydraulic impact drilling devices are often designed to perform a drilling function using a hammer having the same cross-sectional area as that of a narrow shank, where the hammer has the same outside diameter as the shank. However, in hydraulic drilling devices, it is a relatively simple matter to minimize reflected energy losses, as hydraulic drilling devices enjoy the advantage of high pressure hydraulic fluid which can create large forces while acting on a surface of a narrow piston head.
Pneumatic impact drilling devices, by contrast, generally compensate for low working pressures by using large diameter piston heads. Pneumatic drilling devices generally utilize hammers with variable cross-sectional areas that do not enjoy the advantage of minimizing reflected stress waves. Consequently, reflected energy losses are typically significant in pneumatic devices.